If you search this forum you'll find a lot of information about multi-blog scenarios. I've employed a multi-blog approach with Sueetie at
http://sueetie.org. My approach is to use ASPNET Membership and thus SQL Server for user data, but storing all blog data in XML in \app_data. When users are created and the blog configured, the user would be assigned
Editor of the blog and entered in be_roles. I use a site-wide BlogAdministrative role as well. I've documented my implementation pretty thoroughly on the Sueetie Wiki at
http://sueetie.org/wiki/SetupBlog.ashx

I also provided DIFF source with the web.config changes to support ASPNET Membership and SQL script on assigning user permissions on a blog-by-blog basis. That's available on CodePlex at
http://www.codeplex.com/sueetie.